[Tinyos-devel] Lqi and CC2420

[Jeongyeup Paek]

Looking for a QQ plot?
QQ plots collected from Tutornet...

http://enl.usc.edu/~jpaek/data/linktest/prr_lqi_rssi_f4ch23pw3s29i25.html
http://enl.usc.edu/~jpaek/data/linktest/prr_lqi_rssi_f4ch23pw7s29i25.html
http://enl.usc.edu/~jpaek/data/linktest/prr_lqi_rssi_f4ch23pw31s29i25.html

...each with power setting 3,7,31, respectively.

Thanks

- jpaek


On Tue, Mar 4, 2008 at 1:28 AM, Andreas Koepke <koepke at tkn.tu-berlin.de> wrote:
> Philip Levis:
>
> >
>  > On Mar 3, 2008, at 1:00 AM, Andreas Koepke wrote:
>  >
>  >>>>
>  >>>>  Pragmatically: something that delivers a comparable performance.
>  >>> Phil can elaborate more because he has looked at RSSI and LQI in
>  >>> detail but it is not clear if one can get comparable performance using
>  >>> RSSI vs LQI.
>  >>> - om_p
>  >>
>  >> Well, his published results afaik lack a QQ-Plot using the packet
>  >> delivery ratio PDR of RSSI vs. LQI. There may exist a cost function
>  >> such that PDR = cost(RSSI) = cost'(LQI). Theoretically, RSSI and LQI
>  >> must be strongly correlated in the absence interferers (both in system
>  >> and out of system like microwave ovens). So I believe that there is
>  >> such a function, and I would not be surprised if it is linear at least
>  >> for the relevant interval. Excluding interferers is the difficult part
>  >> here...
>  >
>  > I'm not sure why a QQ plot would help. The two functions are very
>  > different: while they fact that they measure the same thing means they
>  > are correlated, that does not mean they have similar probability
>  > distributions, as the LQI/RSSI plots show. Much of the data from the
>  > published results is online, though, so someone can do a QQ plot if they
>  > want.
>
>  I wanted to use PDR as the prob. function, but given your next
>  explanation it would indeed be of limited use.
>
>
>  > There is no accurate function f() such that f(RSSI) = LQI. Two reasons.
>  > First, the quantization of RSSI readings. LQI readings are essentially
>  > lower-order bits on the SNR curve. A one dB change in RSSI can lead to a
>  > 40+ unit change in LQI. Second, LQI represents SINR, while RSSI is
>  > independent of the Interference and Noise.
>
>  Phew -- quite sensitive. So the linear interval would be very small...
>
>
>  > Basically, the CC2420 chip correlation indicator provides fine-grained
>  > information on the part of the SINR curve where the slope is
>  > significant. This is why the values show much larger temporal variation:
>  > a tiny shift in SINR causes a change in LQI. Also, LQI, due to its
>  > measurement, is more greatly affected by statistical sampling. Even with
>  > a very stable RSSI and SINR, you'll see LQI variations.
>
>  Thanks Phil.
>
>
>
>  > Phil
>  >
>
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-- 
Jeongyeup Paek
Ph.D. student
Embedded Networks Laboratory
Department of Computer Science
University of Southern California
http://enl.usc.edu/~jpaek